Alkyd resins are one of the most ubiquitous binders used for ambient-cure, solvent-based coatings. The mechanical and chemical resistance properties of traditional solvent-borne alkyd resins are developed via autooxidative crosslinking of the alkyd film. Alkyd resins can be prepared by the esterification or transesterification of polybasic organic acids or anhydrides, polyhydric alcohols and unsaturated fatty acids or the corresponding oils. The types and relative amounts of the alkyd components largely determine the physical properties of the paint or coating film which the resin forms. Crosslinking occurs when the activated methylene groups in the unsaturated fatty acids or oils of the alkyd are oxidized in air to the corresponding hydroperoxides which subsequently decompose to generate free radicals, thus resulting in an oxidative crosslinking process.
Because of the high molecular weight, the architectural structure and the presence of hydrogen bond forming and accepting functionalities, the intrinsic alkyd viscosity is relatively high and consequently cutting solvents are required to obtain a practical processing viscosity. Traditional liquid alkyd paints contain as much as 30 wt % or more of volatile organic solvent which is usually a hydrocarbon liquid such as white spirit, hydrogenated and low aromatic white spirit. Typically, volatile organic compounds (VOC's) have a boiling point of up to 250° C. measured at a pressure of 101.3 kPa. The environmental burden and the health problems caused by solvent emission impose the development of high solids or even solvent free coatings. The directory 2004/42/EG of the European Parliament sets limits to the maximum concentration of VOC's in categorized paints and varnishes and vehicle refinishing products. The regulatory restrictions have induced research towards new technologies directed at reducing VOC content of coating technologies in a variety of industries.
WO2008/101722 discloses grafted autoxidisable polyester resins showing low viscosities at relatively high average molecular weight, which are highly advantageous in high solid coating formulations. WO2009/062996 describes an autoxidisable coating composition comprising an autoxidisable alkyd resin and an autoxidisable material, wherein said components have a specific molecular weight distribution, which has favorable properties even for high average molecular weights. EP1608694 is related to a method for the preparation of high solids urethane alkyds. WO2007/074333 discloses a coating composition comprising an alkyd resin and a reactive diluent. The reactive diluent is obtained by reacting a polyunsaturated fatty acid with a dicarboxylic acid and a polyol. WO2007/113146 relates to a coating composition comprising an oxidatively drying binder and a reactive diluent. The diluent is an ester of a polyunsaturated alcohol, which can be selected from the group comprising geraniol, nerol, citronellol, and farnesol.
However, while the move to reduced organic solvent-based compositions brings health and safety benefits, these lower VOC coating compositions must still meet or exceed the performance standards expected from solvent-based compositions. Coating compositions with a low content of volatile organic compounds exhibit considerable shortcomings as low rate of drying at temperatures below 30° C., excessive brush drag, more sensitive to forming a film of a patterned appearance (‘wrinkling’), less satisfactory yellowing resistance and impeded hardness development. Modifications have been made to alkyd resins in an attempt to address such concerns. Alkyd resins have been developed that next to autoxidative curing also may crosslink at ambient temperature by a second crosslinking mechanism. Such secondary curing mechanisms may be ensured by providing the autoxidisable alkyd resin with functional groups that may impart further crosslinking, resulting in an even faster drying process of the coating composition. The functional groups may be attached to the alkyd resin by means of any well-known reactions.
The oxidative crosslinking process is usually accelerated by adding driers. The most widely used driers are cobalt carboxylates because of their good drying performance at ambient temperature and coloristic properties. However, recent studies have shown that several cobalt carboxylates including cobalt octoate are considered as very toxic to aquatic organisms and there are indications that cobalt carboxylates may induce carcinogenic effects by inhalation as by analogy with the more water soluble inorganic salts such as cobalt sulfate and chloride. Hence there is an increasing demand for alternative, non-cobalt based driers.
None of the cobalt-free drier systems disclosed in the art has, however, reached or exceeded the performance of the conventional cobalt containing driers under conditions of low temperatures and regarding through hardening in coating compositions. There is still a need to provide cobalt-free alkyd resin systems having a markedly reduced solvent content or being solvent free and showing a similar or improved drying performance. However, the curing agents based on alternative complexes do not show a comparable drying activity to cobalt salts under conditions of low temperatures or do not show a comparable activity with regards to through hardening.
Thus, there exists a need to improve the drying performance of non-cobalt or low-cobalt coating compositions. Furthermore, there exists a need for coating compositions which provide faster drying properties than cobalt-comprising coating compositions. Furthermore, there exists a need for coating compositions which provide fast drying properties under adverse conditions, such as at low temperatures and in the dark.